Sea surface temperature measurements started in 1920 at a number of coastal stations. In the late 1950s, the net was expanded with two ferry lines. In 1972, the observation net was greatly expanded to include pilots, light-ships, voluntary observing ships and ships of opportunity. From 1973, charts covering the entire Baltic Sea, the Kattegat, the Skagerrak (to E09) and lake Vanern are published twice a week. The database contains temperature measurements from 1973 collected on 120 stations with different frequencies and periods.